A variety of high temperature processes are known which employ monolithic catalyst structures to promote the desired reactions, for example partial oxidation of hydrocarbons, complete oxidation of hydrocarbons for emissions control, catalytic mufflers in automotive emissions control and catalytic combustion of fuels for further use in gas turbines, furnaces and the like. Typical of such catalytic systems are the catalysts used in thermal combustion units for gas turbines to provide low emissions and high combustion efficiency. To achieve high turbine efficiency, a high gas temperature is required. This, of course, places a high thermal stress on the catalyst monolith employed, which is typically a unitary or bonded metallic or ceramic structure made up of a multitude of longitudinally disposed channels for passage of the combustion gas mixture, with at least a portion of the channels being coated on their internal surfaces with a combustion catalyst.
In addition to high thermal stress, the high gas flow rates characteristic of combustion units in gas turbines place a significant axial load or force on the catalyst structure pushing in the direction of the gas flow due to the resistance to gas flow, i.e., friction, in the longitudinally disposed channels of the catalyst structure. For example, if a multistage monolithic catalyst structure such as that described in U.S. Pat. No. 5,183,401 to Dalla Betta et al. is employed as a 20 inch diameter catalyst in a catalytic combustion reactor where air/fuel mixture flow rate is about 50 lbs/second at a pressure drop through the catalyst of 4 psi, the total axial load on the catalyst would be about 1,260 lbs.
The combination of exposure to both high temperatures, e.g., temperatures approaching and even exceeding 1,000.degree. C., where metallic monoliths begin to lose strength, and the aforesaid large axial loads (from high gas flow rates) can cause significant movement or deformation of the catalyst support. In fact, in cases where a corrugated metal foil catalyst monolith is used in which the corrugated foil is rolled together in a non-nesting fashion to form a cylindrical, spiral structure in which the foil layers are not bonded together, the combined high temperature and large axial load from high gas flow can cause the whole structure to telescope in the direction of gas flow, particularly when the axial force exceeds the foil-to-foil sliding resistance in the wound structure. Hence, there is a need to provide a support for the catalyst structure to secure it from movement and/or deformation along its axis in direction of gas flow by means of a support structure which will provide the necessary support at high temperatures without interfering with the efficiency and effectiveness of catalytic combustion as a source of motive force for a gas turbine.
In co-pending U.S. patent application Ser. No. 08/165,966 to Dalla Betta et al. filed on Dec. 10, 1993 (Attorney Docket No. P-1065), the use of internally cooled support struts or bars at the outlet to the catalyst structure is described as a means to support the catalyst. This approach has the advantage that the support struts are cooled by air or other heat transfer medium and for this reason the support struts can have high strength against axial loads even at very high temperatures. However, this approach has the disadvantage that the support struts require a source of cooling air and this results in a more complicated combustor system design or requires the use of high pressure air that may not be available in the gas turbine machine. An additional disadvantage is that the air cooled struts are rather widely spaced over the face of the catalyst. This results in high local contact forces or stresses. In certain portions of the catalyst design, these contact forces can exceed the yield strength of the thin catalyst foil resulting in deformation of the foil. This would dearly not be a desirable result and would detract from usage of the air-cooled support struts in high axial load applications.
One possible solution to the foil deformation problem is to provide more cooled support bars so that the contact stress at the outlet face of the catalyst is reduced. However, since the air cooled support bars are rather thick, the use of large numbers of these at the catalyst outlet will increase the blockage to gas flow and increase the overall pressure drop in the combustor system, which is undesirable. Also, the spacing of the air cooled bars would have to be very close to decrease the contact stress with the catalyst foil.
Another possible approach is to use an uncooled metal support. This would allow the support bars to be much thinner in cross section and reduce the total cross-sectional area and the resulting pressure drop. However, this also has a conceptual problem in that the conventional thinking is that at the high operating temperatures of these systems, most metals have greatly reduced strength and would not be able to support the axial load without using a very thick material resulting in high blockage of the gas flow.